1. Field of Invention
This invention relates to the field of laundry washing machines. More specifically, the invention comprises a method and apparatus for measuring load imbalance in the spinning drum of a washing machine, and then using the value of the load imbalance to calculate the maximum safe spinning speed during the water extraction cycle.
2. Description of Prior Art
Laundry washing machines typically use a rotating drum to agitate the clothes being washed. Turning to FIG. 1, which contains cutaways to aid visibility, washing machine 10 has drum 12, which rotates around horizontal axis 14. Clothing load 18 is contained within drum 12. After clothing load 18 has been taken through the washing and rinsing cycles, it is necessary to remove excess water before the clothes can be removed and placed in a dryer. This goal is typically accomplished by rotating drum 12 at a relatively high speed, so that centrifugal acceleration forces clothing load 18 against the interior surface 20 of drum 12. As the rotation of drum 12 is continued, the water within clothing load 18 flows out through perforations in interior surface 20, and is removed via channeling means within drum 12 (not shown).
While many methods are employed to ensure even distribution of clothing load 18, load imbalance is a frequent problem. If clothing load 18 is not evenly distributed, the resulting imbalance will cause a vibration while drum 12 is spinning. If the imbalance is significant, this vibration can cause the rotating drum 12 to strike chassis 16, resulting in damage to the machine. Thus, the detection of an imbalanced load is important for safe operation of washing machine 10.
Several methods have been previously used to detect an unbalanced condition. First, mechanical limit switches (xe2x80x9ctremblerxe2x80x9d switches) can be mounted on chassis 16 to detect an unbalanced load. If sufficient vibration builds, the xe2x80x9ctremblerxe2x80x9d switch will make contact and the resulting circuit is used to trigger a shut-down of the machine.
The same result can be accomplished with an electrical accelerometer switch. This type of device measures oscillating acceleration (vibration) by measuring the mechanical force induced in a load cell. Like the trembler switch, it sends a shut-down signal if a fixed vibration threshold is exceeded.
Yet another method of detecting load imbalance is to monitor the variation in drive motor load when drum 12 is rotated at low speed. FIG. 2 shows a simplified rear view of washing machine 10. Drum pulley 22 is attached to the rear of drum 12. Drive motor 28 is mounted to chassis 16, in the area below drum 12. Drive motor 28 has motor pulley 24, which drives drive belt 26. Drive belt 26, in turn, drives drum pulley 22, which drives drum 12. An imbalanced load in drum 12, will therefore cause a variation in the load experienced by drive motor 28. In order to understand this phenomenon, the reader""s attention is directed to FIG. 3.
FIG. 3 shows a front view of washing machine 10, again in simplified form. The imbalanced load is represented by a single unbalanced mass 30. Drum 12 is spinning in the direction indicated by the arrow. When unbalanced mass 30 is in the position depicted in FIG.3, the gravitational force on unbalanced mass 30 (Fw), opposes the driving torque of drive motor 28, thereby increasing the load. When unbalanced mass 30 is in the position depicted in FIG. 4, the gravitational force acts in the same direction as the driving torque, thereby decreasing motor load. The result is a sinusoidal variation in motor load, resulting from the raising and lowering of unbalanced mass 30 within the earth""s gravitational field. The reader will appreciate that this phenomenon is only observed in washing machines having an off-vertical spin axis. For a machine having a purely vertical spin axis, there will be no load variation caused by gravity.
The magnitude of the load variation within drive motor 28 is proportional to the magnitude of unbalanced mass 30. Thus, if the load variation can be accurately sensed, the magnitude of the imbalance can be determined. The variation in motor load will cause a small variation in motor speed. If drive motor 28 is equipped with an accurate tachometer, it is possible to measure this variation in speed, and it is therefore possible to calculate the magnitude of the imbalanced load. This magnitude is then used to determine whether the load is sufficiently well balanced to initiate the spin cycle. This method is typically employed at a relatively low spin speed in order to detect any imbalance before the vibration has built to a dangerous level. If the load is sufficiently well balanced, drum 12 would then be accelerated to the speed normally used during the spin cycle.
All of these methods, consisting of the trembler switch approach, the accelerometer approach, and the motor load sensing approach, traditionally result in a xe2x80x9cGO/NO-GOxe2x80x9d decision on the spin cycle. If clothing load 18 is sufficiently balanced, the machine will proceed to the spin cycle. If clothing load 18 is not sufficiently balanced, several things may occur. Many machines are programmed to stop and then begin a series of motions intended to redistribute the load. Other machines will simply shut down and await operator intervention. Even for those machines with provisions for an attempted redistribution, the redistribution will only be attempted a few times before the machine shuts down. The result is that a significantly imbalanced load will cause the machine to shut down before the spin cycle, meaning that clothing load 18 will be left soaking wet. The operator often discovers the machine in a seemingly inoperative condition and, unaware that it needs to be reset, places a needless service call. Additionally, the three approaches described require the use of an extra sensor or sensors, thereby adding cost and reliability concerns.
A more sophisticated solution is described in U.S. Pat. No. 5,161,393 to Payne et. al. (1994). The Payne device seeks to calculate the load imbalance, and then use this value to select among several available terminal spin speeds in order to ensure that a maximum permissible vibration is not exceeded. It calculates the load imbalance in a two-step process. First, the device applies a fixed torque to the spinning drum at relatively low speed (approximately 30 to 50 rpm) and measures the time interval required to accelerate the drum to 250 rpm. This time measurement is used to calculate the moment of inertia of the load within the drum, and thereby obtain an approximate value for its mass. The reader should note that, over this relatively low speed range, the time interval is not significantly sensitive to load imbalance; i.e., an imbalanced load will accelerate at nearly the same rate as a balanced one. Thus, the first time interval is measured to determine mass, irrespective of imbalance.
As the drum is accelerated past 250 rpm, a significant load imbalance will retard the acceleration of the drum. This phenomenon is illustrated by FIG. 29 in the Payne et.al. disclosure. An unbalanced load will take longer to accelerate from 250 to 600 rpm, as shown by the diverging angular velocity curves. This information, when used in conjunction with the total load information obtained during the acceleration from low speed to 250 rpm, is used to determine the imbalance. The magnitude of the imbalance is then used to determine what maximum spin speed will be selected from among several discrete available speeds.
The Payne et.al. invention does require reasonably accurate measurement of drum speed and elapsed time. These requirements do not necessarily necessitate additional sensors, however. The reader will note from the Payne et.al. disclosure that the spinning drum is directly coupled to an electric drive motor. The motor controller would typically have time and motor speed sensing means. Thus, by monitoring existing functions of the motor controller, it is possible to determine drum speed and elapsed time without the need for additional sensors. The reader will therefore appreciate that the methodology disclosed in Payne et.al. can be implemented without additional sensors.
The Payne et.al. method is not without its limitations, however. It is not capable of measuring the load imbalance with sufficient accuracy to determine precisely what the terminal spin velocity should be. Rather, it is only capable of measuring the imbalance with enough accuracy to determine whether the load will accelerate smoothly through one of several natural frequencies inherent to the machine. The possible terminal spin speeds are shown in FIG. 28 of the disclosure. This accuracy limitation was acceptable in its field of application - primarily residential washing machines. However, a method of more accurately determining load imbalance so that a continuously variable terminal spin speed could be calculated, is certainly preferable.
The known methods for dealing with load imbalance in a laundry washing machine are therefore limited in that they:
1. Require additional sensors, thereby adding cost to the machine;
2. Provide only a xe2x80x9cGO/NO-GOxe2x80x9d decision on the spin cycle;
3. Result in a machine shut-down, with consequent needless service calls; and
4. Do not provide enough accuracy in the measurement of the load imbalance.
Accordingly, several objects and advantages of the present invention are:
(1) to measure the imbalance in the spinning load without the need for additional sensors;
(2) to provide adjustment of the terminal spin speed over a continuous range, rather than choosing from a few discrete spin velocities;
(3) in the event of a significant load imbalance, to provide for a reduced terminal spin speed, rather than a machine shutdown; and
(4) to measure the load imbalance with sufficient accuracy to calculate the appropriate terminal spin speed.